In general, forklift trucks are used to raise or lower freight or carry freight to a desired location within a restricted space, and are largely classified into engine forklift trucks and electric forklift trucks according to power sources. Electric forklift trucks use electric power supplied from a battery as a power source, and are mainly used in an interior work as they emit no exhaust gas and generate little noise as compared with engine forklift trucks.
An electric forklift truck is manufactured by removing an engine and a fuel tank from an engine forklift truck and installing a drive motor, a hydraulic motor, and a battery, and the drive motor and the hydraulic motor are driven by the battery so that the electric forklift truck can be steered and driven or a working machine can be driven by a hydraulic oil discharged from a hydraulic pump driven by the corresponding motor.
FIG. 1 is a hydraulic circuit diagram of an energy reclaiming system for an electric forklift truck according to the related art.
As illustrated in FIG. 1, the electric forklift truck according to the related art includes an electronic device 1, a hydraulic pump/motor 2, a main control valve unit 20, a lift cylinder 3, a tilt cylinder 4, a side shift cylinder 5 and a reach cylinder 6.
Here, the electronic device 1 is operated by a motor when the working machine is raised and is operated by a power generator when the working machine is lowered. The hydraulic pump/motor 2 is adapted to selectively or simultaneously perform two functions of a hydraulic pump and a hydraulic motor, and is operated by power from the electronic device 1 and selectively supplies a working fluid from an oil tank 7 to the lift cylinder 3, the tilt cylinder 4, the side shift cylinder 5, and the reach cylinder 6 through the main control valve unit 20.
The main control valve unit 20 includes a lift valve 21 configured to control a flow of a working fluid supplied from the hydraulic pump/motor 2 to the lift cylinder 3 and installed in a working fluid supply line 25, and a flow amount limiting valve 22 configured to control a flow of the working fluid discharged from the lift cylinder 3 to an inlet port of the hydraulic pump/motor 2 and installed in a working fluid return line 25a. A check valve 8 is installed in the working fluid supply line 25 connecting the oil tank 7 and the hydraulic pump/motor 2, and the working fluid return line 25a is connected to the working fluid supply line 25 between the hydraulic pump/motor 2 and the check valve 8.
The main control valve unit 20 includes a first control valve 23 configured to control a flow of the working fluid supplied from the hydraulic pump/motor 2 to the tilt cylinder 4 and the side shift cylinder 5, respectively, and to control a flow of the working fluid returning from the tilt cylinder 4 and the side shift cylinder 5 to the oil tank 7, respectively. The main control valve unit 20 further includes a second control valve 24 configured to control a flow of the working fluid supplied from the hydraulic pump/motor 2 to the reach cylinder 6 and to control a flow of the working fluid returning from the reach cylinder 6 to the oil tank 7.
A direction control valve 9 configured to control a flow of the working fluid supplied from the hydraulic pump/motor 2 to the tilt cylinder 4 and the side shift cylinder 5 through the first control valve 23, respectively, and to control a flow of the working fluid returning from the tilt cylinder 4 or the side shift cylinder 5 to the oil tank 7 through the first control valve 23 is installed in the working fluid supply line 25 connecting the tilt cylinder 4 and the side shift cylinder 5, and the main control valve unit 20.
Meanwhile, the lift cylinder 3 is raised by the working fluid supplied through the hydraulic pump/motor 2 by opening the lift valve 21 of the main control valve unit 20 to operate the working machine (or fork carriage). When the lift cylinder 3 is lowered by opening the flow amount limiting valve 22 of the main control valve unit 20, mechanical energy of the working fluid discharged from the lift cylinder 3 operates the electronic device 1 through the hydraulic pump/motor 2 to be converted into electric energy while charging the battery (not shown). That is, the working fluid return line 25a is connected to the working fluid supply line 25 between the hydraulic pump/motor 2 and the check valve 8 so that the working fluid discharged from the lift cylinder 3 through the flow amount limiting valve 22 is not discharged to the oil tank 7 but is guided to the hydraulic pump/motor 2 to reclaim the lowering energy of the working machine (or fork carriage) as described above. The flow amount generated when the lift cylinder 3 is lowered to lower the working machine (or fork carriage) is used as a working flow amount when the working machine or the secondary consumer (for example, the tilt cylinder 4, the side shift cylinder 5, or the reach cylinder 6) is raised.
However, in the energy reclaiming system for an electric forklift truck according to the related art, it is necessary to limit a speed of the electronic device 1 to prevent cavitations generated when the suctioned flow amount is insufficient as compared with the flow amount discharged from the hydraulic pump/motor 2, in the case where the return flow amount generated when the working machine (or fork carriage) is lowered is smaller than the flow amount consumed by a secondary consumer (the tilt cylinder 4, the side shift cylinder 5 or the reach cylinder 6), which lowers energy reclaiming efficiency.
In addition, in the energy reclaiming system for an electric forklift truck according to the related art, when the working machine (fork carriage) is lowered and the secondary consumer is operated at the same time, it is inevitably necessary to limit the speed of the electronic device 1 to prevent cavitations generated when the suctioned flow amount is insufficient as compared with the flow amount discharged from the hydraulic pump/motor 2, which lowers the speed of the working machine of the secondary consumer.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.